Basic ®broblast growth factor (bFGF) regulates diversi®ed biological functions in rat Sertoli cells. This report demon-strates that bFGF inhibits steroidogenesis in developing rat Sertoli cells. Follicle stimulating hormone (FSH)-stimulated estradiol production was reduced by bFGF. Moreover, the amount of cytochrome P450 aromatase, responsible for the irreversible transformation of androgens into estrogens, is decreased by bFGF at the transcriptional level.